I have heard community described as concentric circles. The innermost is filled with those who are actively involved.
Each successive ring represents a lesser degree of involvement. In a synagogue, for example, can a person who only comes once a year to High Holiday services still be considered a community member? Is membership defined by self-identification or some quantitative measure of involvement?

Some research seems to suggest that in spite of the increase in social interaction through media like Facebook and Twitter, teens and young adults are having more difficulty relating to others.